BP/W and Steel Tanks

[nereas]

Darn... when I dove my BP/W with my steel HP120 wet I still needed 36# of weight. Some of us just float like a butterfly(fish) and sting like a bee.

A FAT bee.

 

[Puffer Fish]

36 lbs wet...are you sure??

I regularly dive a 5mm with a AL B/P and an X8-119 and I only need 4 lbs to hold a safety stop at 15' (w/tank down to 500 psi).

He speaks the truth.. I am in the same boat on that one, except SS plate took a couple lbs off. Ok, it is a douple layer 7mm... but still.

 

[wfmiller]

I dive a steel 119 tank and a steel BP and require no weight diving wet with full 7mm and 7mm core warmer, boots, hood and gloves. The same for when I go dry, no weights required.....

It's a beautiful thing : )

 

[Bubbletrubble]

Ahhhh. The complicated question of weighting. The complete answer to your question would "dive" into discussions about doing a proper weight-check, ditchable vs. non-ditchable weights, elements of a BP/W setup, and redundant buoyancy devices. I'll mention a couple of them.
First of all, I think you're really going to like a BP/W setup. It puts weight in an ideal position for good trim, doesn't squeeze you upon inflation, allows you to take some lead off of a weightbelt, can be adjusted to fit you like a glove, stores like a dream, and packs down amazingly well for traveling (esp. to fun places in the Caribbean). What's not to like about it?
I think part of your question could be answered by referring to the tank buoyancy characteristics found on Tech Diving Limited's cylinder specs webpage:
http://www.techdivinglimited.com/pub/tanks.html
There are sooooooo many different tanks out there! Whenever I plan to dive a new tank, I can always figure out how much more/less weight I need by comparing my current tank to the new one on the chart.
As others have pointed out, the other part of your question has to do with the weight of your BP/W setup. Stainless steel BPs typically weigh 4-6 lbs. AL BPs weigh about 2 lbs. Deep Sea Supply sells a Kydex BP which is even lighter. As you do more research on the BP/W, you'll find that some rigs require an STA (single tank adapter) and some don't. The STA sandwiches the wing to the BP and provides anchoring straps for your tank. Some manufacturers offer weighted STAs -- STAs that can weigh 6 lbs. on their own. Personally, in my local San Diego waters, I dive a Halcyon BP/W with a 6 lb. OMS STA and a HP100 steel tank. I dive this in either a drysuit (10 lbs. of lead) or a 7mm wetsuit (4 lbs. of lead). I'm also a firm believer in always having a few pounds of ditchable weight, although others may disagree with me on this point.
If you're trying to decide between purchasing an AL tank vs. a steel tank, I think that the dimensions (80 cuft vs 100 cuft), price, and buoyancy characteristics should be considered. When I was first diving, I remember burning through my AL80 in 30 minutes on a 50 fsw max dive. With a 100 cuft. tank, I have 20 cuft. more air to use during a dive. Now that I am more experienced, my HP100 allows me to actually reach my NDLs on a 60 fsw max dive (if I choose to). Before getting a HP tank, make sure that your LDS can fill it correctly. My HP100 tank (Worthington X7-100) only has 100 cuft. of air if it's filled to 3442 psi. I really like my HP100 steel tank because it allows me to take off 6 lbs. of lead that I would normally wear on a weightbelt when using an AL80. An added bonus of my HP100 tank is that it's a little shorter than my AL80 tank, which makes carrying the HP100 tank much easier even though it's definitely heavier. (Warning: if you have gorilla-like loooong arms, you may find little difference in carrying the tanks.) Unfortunately, my HP100 steel tank was 3x more expensive than my AL80. Last thing to think about is what size tank your dive buddies will be using. If you have about the same air consumption as your dive buddies, it makes no sense to be diving a HP120 when they're diving AL80s. If you run into this problem, depending on how generous you're feeling, tanks make great Christmas gifts to dive buddies.
Have a great time on your diving adventures...and dive safe!

 

[Life-Is-Good-Diver]

I would say it really depends on your tanks, and keeping everything as close to neatrul as possible. As long as you can either ditch, or swim up your rig, if you don't dive dry,and don't have redundent lift.

I use a halycon B/P and wing, and a 6lb SS backplate, but I still have a few pounds of ditchable weight so in a emergency, i can dump and swim up when I'm using my steel 100.



When diving a alum 80, I need approx 4 more lbs of ditchable weight , because the swing on the alum 80's end up around 6 lbs positive when at 700 psi

To be honest, i may drop down to a 2 lb alum BP, just so when I'm diving my
steel 100's, I have alittle more ditchable weight, only because I'm not adding much in my pockets.

I think the person before me hit it right on the nose. The tanks you'll mostley be using have a major factor, because steel 100's start neg 8 lbs, and end up neg 1 pound, while alum 80's start around 2 lbs neg, but end up around 6 -7 pounds positive when they are below 1000 psi.

Good luck with your new suit also, and have fun on your cruise coming up!

 

[PerroneFord]

... while alum 80's start around 2 lbs neg, but end up around 6 -7 pounds positive when they are below 1000 psi.

Could you please tell me which models of AL80s end up 6-7 pounds positive below 1000psi? I'd like to avoid those if I can.

Thank you.

 

[Bubbletrubble]

I could have titled this post: "I have too much time on my hands and I want to think about diving." Translation: I'm a .

The tanks you'll mostley be using have a major factor, because steel 100's start neg 8 lbs, and end up neg 1 pound, while alum 80's start around 2 lbs neg, but end up around 6 -7 pounds positive when they are below 1000 psi.

I'll preface this comment by saying that FLTEKDIVER is probably a far more experienced diver than I, and he probably knows far more than I do about scuba gear. Yet, I have the same question as PerroneFord. As far as I know, the two most common AL80 tanks are the Luxfer 80 and Catalina S80. The Luxfer 80 is +4.4 lbs. when empty and the Catalina S80 is +4 lbs. when empty. I have never heard of AL80 tanks that "end up around 6-7 pounds positive when they are below 1000 psi," but what's 3 or 4 pounds between friends, eh?

Emily, I hope that we're not confusing you. Figuring out how much more/less weight you need based on a tank change is really just simple math. I remember when my high school math teacher said that I should pay attention because this stuff might save my life one day! Check out the Tech Diving Limited webpage for the buoyancy characteristics of specific tanks. For this first part, I'll focus on the "Buoyancy Empty" numbers. Let me work through an example for ya:
Let's say that, with a BP/W setup, you wear 14 lbs. of ditchable lead when diving in your brand-new Henderson 7mm Thermoprene wetsuit with a Catalina S80 (and you've done a proper weight check to determine that this is the right amount of weight for holding a 15ft safety stop at 500 psi), but a magnanimous dive buddy is letting you borrow his Worthington X7-100 HP100 steel tank for a dive.
The Catalina S80 is +4 lbs. buoyant when empty, whereas the X7-100 is -2.5 lbs. buoyant when empty. Thus, when empty, the X7-100 is 6.5 lbs. heavier than the Catalina S80. This means that you can subtract approx. 6.5 lbs. from your 14 lbs. of ditchable lead. Therefore, you need to wear only 14 - 6.5 = 7.5 lbs. of ditchable lead with your dive buddy's X7-100 tank.
[N.B.: For the nitpicking math/physics jocks in the audience, these calculations are still approximate for the two tanks in question because they assume that the buoyancy differential when empty is equal to the buoyancy differential when at 500 psi.]

You may then wonder: With an empty wing (or a BC failure), will ditching that 7.5 lbs. of lead be enough to make me positively buoyant at the beginning of the dive? Once again, consulting the Tech Diving Limited tank chart, we see that the X7-100 is -10 lbs. buoyant when full and -2.5 lbs. buoyant when empty. Thus, the weight of a full tank of air is 7.5 lbs. at the beginning of the dive. Soooooo, if you ditch your 7.5 lbs. of lead at the beginning of the dive with a full X7-100 tank and an empty wing, you will be...about neutral. But wait, you did your weight check at the 15ft safety stop depth! Your wetsuit loses buoyancy with increased depth. The newer the wetsuit, the more it gets compressed at depth. So, in reality, you'll probably be slightly negative if you ditch your lead at the beginning of the dive at depth while using your buddy's X7-100 tank. Is it too negative to swim up? You'll have to be the judge of that.

For homework...
OK, now let's say that your buddy wants to lend you a Faber HP-100 tank. How much lead would you have to wear? Will ditching the calculated amount of lead make you negatively/positively/neutrally buoyant at the beginning of the dive with an empty wing (or BCD failure)? The answer to this last question might explain why some people make the blanket statement that it's "dangerous" to dive a steel tank with a wetsuit. These safety-conscious folks are probably referring to the fact that if you get into trouble at the beginning of the dive (BC failure) and need to ditch your weights, you may still be negatively buoyant...depending on your depth and gear configuration (ditchable weight, steel tank, compressed neoprene wetsuit). Shell drysuits don't become "compressed" at depth like neoprene wetsuits. Also, drysuits can be pumped full of air and used as another buoyancy control device, thus, making it safer to dive with a steel tank.

OK, grab some coffee , and let's work through one more math problem...
In my original example we said that you did your weight check using your BP/W rig. Let's say that you actually did your weight check using a traditional jacket-style BCD which has an inherent buoyancy of +2 lbs. (as Tobin alluded to in his post). For argument's sake, let's say you went out and replaced your jacket-style BCD with a stainless steel BP and harness that was -5 lbs. buoyant. That's a 7 lb. buoyancy differential, meaning that you would only need 14 - 7 = 7 lbs. of ditchable weight at the beginning of the dive...with an AL80 tank. Using a Catalina S80 tank, the weight of a full tank of air is 5.8 lbs. Therefore, you would only be slightly positive if you ditched your weights with this rig at the beginning of the dive with a full tank. If you decided to dive a HP100 steel tank with this SS BP, you would be about 7.5 lbs. negatively buoyant at the surface at the beginning of your dive with an empty wing...with no ditchable weight. At least you wouldn't have a difficult time sinking! To address this issue, often times divers will match an AL BP with a steel tank and a SS BP with an AL tank.

Full disclosure: I am not a DIR diver. However, I have read that DIR divers (and others) recommend not using a steel tank while diving a wetsuit, especially with a SS BP. Having done the calculations, I can see the value in making the blanket statement: "It's dangerous to dive a steel tank with a wetsuit."


Do I dive a steel tank with a wetsuit and my SS BP? Yes.
Will I be positively buoyant if I have a BCD failure and need to ditch my lead at the beginning of a dive? No.
Am I diving an unsafe gear configuration with a wetsuit + steel tank + SS BP? In my opinion, no...so long as I feel comfortable swimming up the gear from depth given a BCD failure.

I'll let Tobin (a.k.a. cool_hardware52) or someone else do the calculations to determine which size wing you'll need to buy based on your gear configuration. You basically want a wing that has enough buoyant lift to float your gear (excluding exposure suit) when it's at its heaviest (with a full tank of air).

OK, my typing fingers are going numb. I'll stop. If you read all the way to the end of this post, you deserve to be entertained by an animation of me dancing.

 

[dannobee]

... because steel 100's start neg 8 lbs, and end up neg 1 pound, while alum 80's start around 2 lbs neg, but end up around 6 -7 pounds positive when they are below 1000 psi.

The amount of air in the tank determines the "swing." 100 cu. ft. of air weighs about 7.5 lbs. If that AL80 swings 8-9 lbs, then somebody managed to squeeze 130 cubic feet of gas in it.

The "swing" on ANY 80cu ft tank is about 6 lbs.

 

[John_B]

I'm new to doubles myself, but the only way FLTEKDIVER's numbers make sense to me is if he was talking about doubled Al80s. But then the steel numbers don't make any sense...



John

 

[Life-Is-Good-Diver]

Sorry, to catch up so late, the regular scuba tank single cylinder has a swing going from about
-1.4 to a positive swing of about +2.5 at 500 psi, and when empty it's +4.4 positive, which none of us will be breathing underwater, a empty tank See here :
http://www.mdaguam.com/tankspecs.htm

Sorry, i was comparing my double alum 80's, as when i dive them in doubles, I'm around 5lbs more positive when both tanks are at 500psi.

I have a chart with my weight I follow, so when I'm diving double steel 100's , I'm way to heavy with my 6lb BP, as the swing on my steel 100's , when there full I'm at -10 lbs for each tank, and when there empty, they have a rating of -2.5 lbs , so i figure around -4 lbs per tanks , plus a 6lb BP, and that puts me at 14lbs of non ditchable weight. Plus there are other considerations, such as dry-suit, like my TLS-350 , verse a wetsuit, etc, weighting is tough when you have multipule set up's to dive with.

Going back to a single tank, like a alum 80, you'll end up from a neg -1.4 to a +2.5 , that you will have to compasate for.

You can add all the numbers up you want, but what I did, and this is part of the Padi tech corse, is to swim up your rig with the B/C fully deflated, and see if you can swim it up. I started off years ago way negitive, and learned alot over the years to be as close to neutral as possible.

Then you have to take into consideration, redundent lift, which is another whole topic.

Sorry for the mis-leading 6-7 lbs before, I jump on and off these threads pretty quiclkey these days, as I have so much going on, summers all most over, lol

Have fun